Heretofore, an instrument called a plethysmograph has been used for measuring changes in the size of a part of the body by measuring changes in the amount of blood in that part. FIGS. 2A and 2B show two examples of the instrument.
In use of one instrument, disposed around a limb, as shown in FIG. 2A, an arm 1 in this case, a pair of current applying wire electrodes 2a and 2b and a pair of voltage detecting wire electrodes 3a and 3b are wound. A predetermined alternating current is applied to the current applying wire electrodes 2a and 2b. Voltage between the voltage detecting wire electrodes 3a and 3b is measured for detecting impedance or admittance between the wire electrodes 3a and 3b. When the volume of the arm 1 between the voltage detecting wire electrodes 3a and 3b becomes large, the impedance there-between decreases, or the admittance increases. Thus, the volume of the body is detected in accordance with a measured value of impedance or admittance.
In use of the other instrument, as shown in FIG. 2B, a cylindrical tank 4 is provided. The tank 4 has an inner flexible tubular member 6 made from a rubber film. End rims of tubular member 6 are supported by means of flexible end flanges 5a and 5b which are fixed to the tank 4. The space between the tank 4 and the inner tubular member 6 is filled with a predetermined amount of water W. When the arm 1 is thrusted into the tubular member 6, the member 6 is deformed dependent on the size of the arm 1. Thus, a a portion of the water W disposed about the tubular member 6 overflows through an overflow pipe 7. The amount of the water overflow is measured to obtain the volume of the arm 1.
However, in the case of the former instrument, since wire electrodes are directly attached to the arm 1, high voltage cannot be applied to the current applying wire electrodes 2a and 2b in order to prevent injury to the body. Accordingly, it is impossible to improve the accuracy in the measurements. Furthermore, the operator must be skilled in attaching the wire electrodes to the body.
In the latter case, flexible flanges 5a and 5b are liable to project outwardly as a result of the pressure of the water when the arm 1 is inserted into the tubular member 6, so that the correct amount of water overflowing into the overflow pipe 7 in accordance with the changes of the volume as a result of the arm 1 being inserted cannot be accurately measured.